1. Field of the Invention
The present invention relates to a reflector telescope and particularly, to a catadioptric reflector telescope employing a spherical primary reflector having a relatively short focal length and which reduces or substantially eliminates the problems conventionally associated with the use of spherical mirrors in reflector telescopes.
2. Description of the Prior Art
Reflector telescopes generally employ a concave primary reflector which collects and focuses light incident upon the reflector surface through the telescope's aperture and a smaller secondary reflector to direct the reflected light to the telescope's eyepiece. Normally, the primary reflector is provided by a paraboloidal mirror which has certain advantages in terms of imaging, the most important being that all parallel rays of light incident on the mirror are convergent at the focal point of the mirror, irrespective of the distance of the point of incidence from the central axis of the mirror. The result is that the focussing ability of parabolic or parabolical mirrors is generally good, even if the focal length of the primary mirror is short.
However, the use of paraboloidal mirrors in reflector telescopes does have certain disadvantages. Firstly, the image is strongly distorted towards the edge of the field of view—an aberration of the reflector known as coma in which the image of a point lying off the axis of the reflector has a comet-shaped appearance—making such telescopes unsuitable for photography. Moreover, coma is more strongly pronounced as the focal length of the primary reflector is reduced so that long focal length reflectors must be used if the effect is to be minimised, thus resulting in a longer overall length of the telescope. In addition, the manufacture of paraboloidal mirrors is difficult and expensive.
The above disadvantages are overcome by the use of a spherical primary reflector in place of a paraboloidal one. Spherical mirrors are considerably easier and cheaper to manufacture than paraboloidal mirrors and they do not generate coma distortion and are therefore suitable for photographic purposes.
However, the use of spherical primary reflectors in reflecting telescopes also presents a number of disadvantages. While coma distortion is not present, spherical mirrors suffer from a defect known as spherical aberration in which the rays of light incident on the mirror come to a focus in slightly different positions rather than a common focal point. Full size correctors are therefore normally required to compensate for this defect, as in the case of, for example, the Maksutov telescope and the Schmidt camera. Naturally, this results in manufacturing difficulties and increased expense. Moreover, spherical aberration varies inversely with the cube of the focal length of the mirror so that, again, the manufacture of compact telescopes is extremely difficult. Increasing the focal length of the primary reflector in order to compensate for spherical aberration increases the length of the telescope. These instruments also normally have a very large central obstruction, which degrades the image by transferring some of the energy in the Airey disc to the diffraction rings. Maksutov and Schmidt/Cassegrain telescopes also have a third reflection normally through a hole in the primary mirror and thus have an inconvenient observing position.
In general, therefore, the disadvantages of spherical reflectors outweigh those of paraboloidal mirrors and it is the latter that are therefore more commonly used in reflector telescopes. If the disadvantages associated with spherical mirrors could be reduced or substantially eliminated, however, then the use of spherical mirrors in reflector telescopes would be of considerable advantage.